1. Field of the Invention
The present invention relates to secondary surveillance radar (SSR) for surveying a target such as an airplane equipped with a mode S transponder and an air traffic control radar beacon system (ATCRBS) transponder.
2. Description of the Related Art
An SSR mode S is a radar system to capture a variety of items of information owned by an airplane by transmitting an interrogation from a beam antenna of a device to a transponder mounted on a target such as an airplane and receiving a response (cf., for example, documents “ISBN0-89006-292-7”, “Revision Radar Technique”, issued by Institute of electronics, Information and Communication Engineers, 1996, P. 227-233”). There are two kinds of transponders, which are conventionally used ATCRBS transponder and mode S transponder, and there are a variety of interrogation systems to capture these two kinds of transponders.
In the SSR mode, the SSR divides a beam dwell time into two or more, further divides one of them (referred to as one scheduling period) into an all-call period (period to capture a mode S transponder and an ATCRBS transponder) and a roll-call period (period to transmit selective interrogation to mode S transponder) to capture a target. In a surveillance protocol, the length of a response is 64 (μs) and the length of an interrogation is 19.75 (μs).
And the SSR mode S has a data-link protocol, in this case, the length of replay becomes 120 (μs) and the length of interrogation becomes 33.75 (μs), so that it takes about double of time compared to surveillance. If the data-link protocol is frequently used in the future, there will be a method for simply lengthening the time length of the roll-call period; however, deterioration in detection rate of a conventional ATCRBS transponder will be caused (because of reduction in the number of responses). Therefore, it becomes important how to efficiently capture the target in the beam dwell time so as to capture the mode S transponder while maintaining the detection rate of the ATCRBS transponder.
Conventionally, in the roll-call period, the SSR calculates prediction positions from previous track information of individual airplanes with the mode S transponders mounted thereon and varies a range of an azimuth angle for transmitting an interrogation in accordance with the prediction range among the prediction positions. However, in the case of the short range target, since the range of the azimuth angle for transmitting the interrogation is expanded, the SSR schedules transactions (transmission and reception of interrogation and response) up to the azimuth having a small possibility of receiving the response. And there is a possibility of an occurrence of the problem that the SSR cannot schedule the transactions for the target in a relatively short range within a beam-width of an antenna (generally, indicating an angle range of beam center azimuth ±1.250°), because the SSR schedules transactions prior to a long distance target in the case of congestion of targets at long distances within the corresponding beam.
Furthermore, in the case of use of the data-link protocol, since the length of the interrogation and the length of response become longer than those for surveillance, the time required for the transactions is lengthened and the possibility of an occurrence of an event which cannot be scheduled by the SSR is further increased.